This invention relates to a pressurized screening apparatus for separating fiber suspension, preferably pulp suspension. The screening apparatus comprises a screen housing with an upper chamber highest up in the screen housing, a rotor means, which is rotary about a rotor shaft, and at least two coaxially arranged screen means. A first tubular screen means divides the interior of the screen housing in radial direction into a first screen chamber located outside the first screen means with an outer defining surface, and a first accept chamber located inside the first screen means. A second tubular screen means divides the interior of the screen housing, so that a second screen chamber is formed between the second screen means and an inner defining surface. The first screen means has a smaller diameter than the second screen means and at least partially is located inside the second screen means. The screening apparatus further comprises an inlet means for fiber suspension to the screening apparatus, at least one reject outlet for reject from the screening apparatus, and at least one accept outlet means for accept from the screening apparatus.
Such a screening apparatus with several screen means (screening steps) in the same screen housing is called combi-screen. A combi-screen of the aforesaid type is used, for example, at multistep screening of pulp suspensions, preferably for fractionating or separating impurities and other foreign matter undesired in the final product, such as shives, coarse particles, scrap, stones or incompletely digested or not refined chip bits.
One example of a screening apparatus of the above described type and showing known state of art is set forth in PCT application WO95/06159. The first screen means as well as the second one are stationary. The second screen means divides the interior of the screen housing into a second screen chamber located inside the second screen means, and a second accept chamber located outside the second screen means. The pulp suspension to be screened is introduced via the inlet means to the first screen chamber, where the approved fraction, the accept, flows through the first screen means and into the first accept chamber. In the first screen chamber and near the first screen means, pulse elements, so-called wings, are provided on the rotor means, upon rotation of the rotor means, pulses are created by the wings which assist in guiding the accept through the first screen means and into the first accept chamber. The accept flows up through the first accept chamber and further up through and inside the rotor means to the second screen chamber. In the second screen chamber, as in the first one, wings are located on the rotor in order to create pulses at the second screen means. The accept from the second screen chamber flows through the second screen means and out into the second accept chamber to be discharged via the accept outlet, which is located in the second accept chamber.
The second screen chamber communicates, and partially coincides, with the first screen chamber, so that the pulp suspension (the reject), which does not pass the second screen means, flows down to the first screen chamber. There the reject from the second screen chamber again can pass through the first screen means or flow out through the reject outlet means, which is located in the first screen chamber.
Another combi-screen is shown in PCT application WO96/02700 where a coarse screen is combined with two more screen steps. All screen means are here thought to be located above each other.
Combi-screens have been developed in order to bring about a cheaper screening apparatus than is the case if every screen step is arranged in separate screening apparatuses. Combi-screens, however, tend to be relatively high, because the screen means are combined one above the other. Combi-screens can imply compromises, which in most cases result in poorer screen results and high energy consumption.
The present invention has the object to offer a solution of the aforesaid problems and to indicate a compact combi-screen, which yields good screen results and low energy consumption. The outer defining surface of the first screen is given a greatest diameter which is smaller than the smallest diameter of the inner defining surface of the second screen chamber. The first screen chamber at least partially is located inside the second screen chamber, and the first screen chamber is separated from the second screen chamber. The inner defining surface of the second screen chamber constitutes the inner defining surface of the second screen step. This implies that the first screen step inclusive of the screen means with the chambers outside and inside thereof are located at least partially inside the second screen step. The screening apparatus thereby can be constructed low and becomes lower the greater portion of the first screen step is located inside the second screen step.
In order to achieve a good screen result, it is important that the first screen step yields a uniform pulp quality for subsequent screen steps. In a screening apparatus of the said kind, instead of the first stationary screen means, according to known state of art, a first rotary screen means is provided. Instead of the rotating wings a stator is provided inside the rotary screen means, and on the stator at least one pulse means is located. The first screen means is placed on the rotor means to rotate with the same. Suitably at least one of the pulse means shall be a barrier/pulse element. The barrier/pulse element extends in axial direction substantially along the entire stator and the entire first screen means. Upon rotation of the first screen means the barrier/pulse element creates both suction pulses to guide the accept into the first accept chamber and pressure pulses to clean the first screen means and to prevent thickening and plugging in the first screen chamber.
The rotary screen means is given a considerably smaller diameter relative to the second screen means. Hereby the circumferential speed at screening at the rotary screen means becomes considerably lower than at the second screen means. The energy demand thereby is lower than in the case of an unnecessary high circumferential speed at the screen surface. When the circumferential speed at the screen surface is greater than demanded for the screening, for example, shives and impurities can be worked so that they are disintegrated and, thus, can pass through the screen means. The considerably smaller diameter is a great advantage when the first screen step is a coarse screen step. There one needs to have, and preferably would like to have, a considerably lower circumferential speed at the screen surface than is needed at fine screening.
The characterizing features of the invention are apparent from the attached claims.